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<ul><li><p>2 </p><p>The Role of Aluminum-Organo Complexes in Soil Organic Matter Dynamics </p><p>Maria C. Hernndez-Soriano Department of Soil Science, College of Agriculture and Life Sciences </p><p>North Carolina State University, Raleigh NC USA </p><p>1. Introduction </p><p>The knowledge achieved during the last decades on the dynamics of organic matter (OM) and inorganic elements in soils has been essential to predict long-term effects of land management and develop sustainable practices that contribute to mitigate the decline in soil quality and the potential threats for human health. Knowledge about soil organic matter (SOM) properties is essential to understand soil processes. The distribution and chemical speciation of organic carbon (OC) in soil have a major role on biogeochemical processes, e.g. its own chemical stability and the mobility and bioavailability of nutrient and contaminants (Eusterhues et al. 2005; von Lutzow et al. 2008). Therefore, SOM properties are directly related to essential environmental processes, e.g. plant production, carbon sequestration or water pollution. The decline in SOM quality results in land degradation, increasing flooding events or the rates of irrigation and fertilization necessary for agricultural activities. Organo-mineral associations and complexation of SOM with metals ions largely determines the stability and degradability of OM (Kogel-Knabner et al. 2010). The relevance of inter-molecular interactions of OM with metal ions in solution on substrate degradation, e.g. complexation with aluminum (Al), has been already highlighted in the existing literature (Sollins et al. 1996) and prompted the research presented. </p><p>1.1 Stability of organic carbon in soils </p><p>Overall, this chapter addresses the relevance of understanding the mechanisms responsible for OC stabilization in soil. For instance, knowledge on SOM stability is essential to develop strategies for carbon sequestration in soil. Soil organic matter constitutes approximately 2/3 of the global terrestrial C pool (Batjes 1996), and OC dynamics in soils control a large part of the terrestrial carbon (C) cycle. In general, human activities cause a net release of CO2 to the atmosphere of about 800 Gt C per year (Schlesinger 1984; Schlesinger and Andrews 2000) and more concretely, forest conversion to agriculture can release up to 75% of stored soil OC as CO2 (Lal 2004). Besides, world soils have constituted a major source of enrichment of atmospheric concentration of carbon dioxide (CO2) ever since the dawn of settled agriculture, about 10,000 years ago. Nowadays carbon sequestration in soils is a main area of research because of the importance of soils for food production and its role in the global carbon cycle. Depending on </p><p></p></li><li><p> Soil Health and Land Use Management </p><p>18</p><p>environmental conditions and land use, soils may act as sources or sinks for C. Therefore, understanding the mechanisms that control stabilization and release of C is essential for the prediction of the effects of global climate change and for the development of management strategies to increase carbon sequestration in soils, which constituted a major demand at the Kyoto Protocol on climate change in 1992. In general, it can be assumed that the pool of stable SOM in the soil solid phase is in equilibrium with the soluble organic matter (dissolved organic carbon, DOC) in the soil solution (Fig. 1). The term SOM refers to all organic substances in the soil. Organic carbon in soil can originate from natural or anthropogenic inputs, i.e. plant and animal litter decomposition, substances synthesized through microbial and chemical reactions and biomass of soil micro-organisms, but also soil addition with organic amendments. The turnover of C in soils is controlled mainly by water regimes and temperature, but is modified by factors such as size and physicochemical properties of C additions in litter or root systems or distribution of C within the soil matrix and its interactions with clay surfaces (Oades 1988). The stabilization of organic materials in soils by the soil matrix is a function of the chemical nature of the soil mineral fraction and the presence of multivalent cations in the soil solution (Fig. 1), the presence of mineral surfaces capable of adsorbing organic materials, and the architecture of the soil matrix. The degree and amount of protection offered by each mechanism depends on the chemical and physical properties of the mineral matrix and the morphology and chemical structure of the organic matter. Thus, each mineral matrix will have a unique and finite capacity to stabilize organic matter (Baldock and Skjemstad 2000). Three types of pathways are commonly considered in the formation of stable OM in soils (Christensen 1996; Sollins et al. 1996): Selective enrichment of organic compounds, which refers to the inherent recalcitrance of specific organic molecules against degradation by microorganisms and enzymes (Fig. 1); Chemical stabilization, involving all intermolecular interactions between organic substances and inorganic substances leading to a decrease in availability of the organic substrate due to surface condensation and changes in conformation, i.e., sorption to soil minerals and precipitation; Physical stabilization, related to the decrease in the accessibility of the organic substrates to microorganisms caused by occlusion within aggregates. According to Kogel-Knabner et al. (2008), the protection against decomposition imparted to soil organic carbon (SOC) by these mechanisms decreases in the order: chemically protected &gt; physically protected &gt; biochemically protected &gt; non-protected. Hence, the relationship between soil structure and the ability of soil to stabilize SOM is a key element in soil C dynamics. The sorption of OM is assumed as a chemisorptive process that occurs concomitantly with changes in OM conformation. Organomineral interactions lead to aggregations of clay particles and organic materials, which stabilizes both soil structure and the C compounds within aggregates, but due to the heterogeneity of natural soil systems different adsorption mechanism(s) may operate. Besides, different studies have evidenced the different depth distribution of OC in soils (Kaiser and Guggenberger 2000; Gillabel et al. 2010) and the dissimilarity in controls on C dynamics and decomposition of soil OC with depth in topsoil and subsoil (Fontaine et al. 2007; Salom et al. 2009). For instance, according to Guggenberger and Kaiser (2003) sorptive preservation by location of OM in small pores rarely occur in topsoil horizons but primarily in subsoil horizons. Furthermore, the authors indicated that Fe oxides may be the most important sorbents for the formation of organo-mineral associations in the subsoils, which was later corroborated by Kogel-Knabner (2008). </p><p></p></li><li><p> The Role of Aluminum-Organo Complexes in Soil Organic Matter Dynamics </p><p>19 </p><p>Fig. 1. Conceptual scheme of carbon cycling in soil. </p><p>Chemical protection involves interactions of OM with minerals; physical protection makes OC inaccessible to microbes and enzymes; and biochemical protection results from differential degradability of organic structures. Chemical stabilization may result from association of OC compounds with mineral surfaces primarily in silt- and clay-sized particles (Sollins et al. 1996), but the mechanisms by which the different OM fractions adsorb onto mineral surfaces and the relationship between mineralogy and the chemistry of OM bound in organo-mineral associations are not yet fully understood. </p><p>1.2 Chemical protection of organic matter </p><p>For the purposes of the research discussed, this chapter focuses on the mechanisms involved in chemical stabilization. The primary scope is to provide a comprehensive understanding of chemical stabilization of OC in subsoil, considering the following statements: - Chemical protection of OM might be predictable from soil clay mineralogy and </p><p>extractable forms of Fe and Al (Kgel-Knabner et al. 2008). - Complexation of fresh organic matter with Fe or Al oxides might constitute a major </p><p>mechanism for OM stabilization (Eusterhues et al. 2005), forming aggregates protected from microbial degradation. Moreover, organic matter has been described to be more protected in subsoils than in topsoils (Gillabel et al. 2010). The total amount of C in subsoils is in general larger than in topsoils, but concentrations are in general lower in subsoil. The specific surface area (SSA) available in the subsoil for adsorbing OM is determined by the subsoil mineralogy and therefore the amount of OM that can be stabilized before reaching saturation (Eusterhues et al. 2005). </p><p>1.3 Complexation of aluminum by organic compounds </p><p>Aluminium is the third most abundant element in the Earths crust, occurring at about 8%, and is a main or secondary component of numerous minerals, especially silicates. The only </p><p></p></li><li><p> Soil Health and Land Use Management </p><p>20</p><p>stable ion, Al3+, is known to coordinate with oxygen-bearing ligands (Kabata-Pendias 2011). Metal ions in aqueous solution exist as aqua ions, where water molecules act as ligands, and coordinate to the metal ion via the oxygen donor atoms. Solution properties of Al are complex (Fig. 2), it is present as Al3+, Al(OH)2+ Al(OH2)+ at pH</p></li><li><p> The Role of Aluminum-Organo Complexes in Soil Organic Matter Dynamics </p><p>21 </p><p>Organic matter has been described to floculate with Al salts. Maison et al. (2000) described the existence of specific binding sites for Al for given structures or ligands within the OM composition. Those results suggested that the organic ligands present in the OM are responsible for the distribution of metals. </p><p>1.4 Methodological approaches to characterize aluminum-organic interactions </p><p>The chemical complexity of SOM, the heterogeneity of its distribution and the variations in size and decomposition rate create significant analytical problems and partly explain the current deficiency in our understanding of SOM chemistry and dynamics (Lehmann et al. 2010). Spectroscopic techniques are powerful tools in environmental research and a growing field of research (Schulp et al. 2008; Scheckel et al. 2010). The momentum of synchrotron research is leading to a continuous success of synchrotron studies addressing complex environmental issues and moreover, it can be expected that regulations and policy decisions will increasingly rely on such techniques. Fluorescence spectroscopy has the required sensitivity to characterize metal ion binding properties of organic matter and determine its micromolar complexing capacities, i.e. to allow differentiating the binding sites or the ligand types involved in the formation of metal-organo complexes (Ryan and Weber 1982). Besides, the fact that fluorescence differentiates free from bound ligand provides an excellent complement to other complexing capacity techniques which measure free metal ion, like anodic stripping voltammetry or ion selective electrode potentiometry. Thus, fluorescence spectroscopy has been probed as valuable for the analysis of complexes between humic materials and several metal ions. For instance, fulvic acid exhibits fluorescence that is quenched upon binding to a paramagnetic metal ion. </p><p>2. Spectroscopic analysis to determine the formation of aluminum-organo complexes </p><p>Several methods are available for examining aluminium interactions with natural organic ligand solutions. Spectroscopic approaches allow an accurate characterization of metal-organic complexes. Thus, ultraviolet, infrared, fluorescence and 13C NMR spectroscopy are used to evaluate the functional groups involved in binding, i.e. the aluminium-organic complexes. Fluorescence spectroscopy is well-known as a powerful technique useful for chemical characterization of humic acids while Fourier transform infrared spectroscopy (FTIR) can provide an insight into structural characteristics of complex organic macromolecules and allow typing organic molecules at the micrometer resolution. For instance, FTIR mapping have shown the location of organic C forms in relation to mineral surfaces, and relevant information on microaggregate formation. </p><p>2.1 Ultraviolet spectrophotometric determination of aluminum-organic complexes </p><p>Spectrophotometric titrations with Al(III) were carried out for gallic acid (Figure 3a) and salicylic acid (Figure 3b) to demonstrate the formation of the metal-organo complexes. UVvisible spectra were recorded for increasing concentrations of Al(III). A proportional decrease was observed for the absorbance at 220 and 270 nm for gallic acid (GA) and at 240 and 300 nm for salicylic acid (SA). The two isosbestic points (IUPAC 2007) confirmed the formation of the metal-organo complexes and are indicative of a transition between two light absorbing species in all recorded spectra (Harrington et al. 2010). </p><p></p></li><li><p> Soil Health and Land Use Management </p><p>22</p><p> (a) </p><p> (b) </p><p>Fig. 3. Spectrophotometric titration of a) gallic acid 50 M and b) salicylic acid 50 M with Al(III). </p><p>Titrations were carried out at a pH range 3.914.25, which corresponds with a fraction of GA dissociated of 86-92%, 90-94% for SA and 713% hydrolization of Al(III). Therefore the </p><p>0.0</p><p>0.5</p><p>1.0</p><p>1.5</p><p>2.0</p><p>2.5</p><p>200 250 300 350</p><p>Ab</p><p>so</p><p>rb</p><p>an</p><p>ce</p><p>lambda (nm)</p><p>0</p><p>1</p><p>2</p><p>3</p><p>4</p><p>200 250 300 350</p><p>Ab</p><p>so</p><p>rb</p><p>an</p><p>ce</p><p>lambda (nm)</p><p></p></li><li><p> The Role of Aluminum-Organo Complexes in Soil Organic Matter Dynamics </p><p>23 </p><p>decrease in absorbance can be mostly related with the formation of an inner-sphere complex. The shift of the absorbance maximum at the highest concentrations of Al(III) can be only partially explained by the pH variation and might be largely related to the formation of outer-sphere complexes. </p><p>2.2 Luminiscence spectrophotometric determination of aluminum-organic complexes </p><p>Fluorescence excitation-emission spectra were collected to characterize the formation of aluminium:gallic acid (Al:GA) complexes. Multidimensional fluorescence spectroscopy has been previously demonstrated to provide better characterization of metal binding to organic compounds than traditional quenching at a single excitation-emission wavelength. </p><p> (a) </p><p> (b) </p><p>Fig. 4. Excitation-Emission spectra for gallic acid 50 M (a) and Al:GA 1:1 (b). </p><p>240</p><p>290</p><p>340</p><p>390</p><p>250 300 350 400 450 500</p><p>Ex</p><p>cit</p><p>ati</p><p>on</p><p> (n</p><p>m)</p><p>Emission (nm)</p><p>42</p><p>84</p><p>168</p><p>293</p><p>377</p><p>419GA</p><p>240</p><p>290</p><p>340</p><p>390</p><p>250 300 350 400 450 500</p><p>Excit</p><p>ati</p><p>on</p><p> (n</p><p>m)</p><p>Emission (nm)</p><p>26</p><p>79</p><p>132</p><p>185</p><p>237</p><p>264Al: GA </p><p>1:1</p><p></p></li><li><p> Soil Health and Land Use Management </p><p>24</p><p>Aluminum complexation with GA varied the fluorescence fingerprinting and therefore provided information on the types of metal-organic complexes, which depended on Al(III) concentration in solution (Ohno et al. 2008). Spectral variations confirmed the formation of metal-organo complexes (Figure 4) but furthermore suggested the formation of at least two different types of complexes (Figure 4b). Similarly, for the titration of glucose (Glu) w...</p></li></ul>


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